Fused silica radome

ABSTRACT

A new and improved fused silica radome is discussed. The radome has a density equal to or greater than 95 percent of theoretical density, an average bend strength equal to or greater than 10 Kpsi, a thermal expansion of about 0.3×10 -6  ° C., a dielectric constant of about 3.8 at X-Band frequencies, a loss tangent of about 0.002 at X-Band frequencies, an average modulus of rupture greater than 10 Kpsi from room temperature to 1000° C., and no crystalline phase present. The radome is made by an arc fusion process in which a quartz powder is placed within a graphite mold, then shaped by centrifugal force as the mold is rotated. An arc is then struck between electrodes within the mold cavity. The quartz powder fuses to form a dense silica radome which is removed from the mold after the fusion occurs.

FIELD OF THE INVENTION

This invention relates to a silica article of manufacture. Moreparticularly, this invention relates to a silica radome.

BACKGROUND OF THE INVENTION

This invention concerns radomes. Examples of radomes are disclosed inU.S. Pat. Nos. 4,615,933, 4,615,859, 4,522,883, 4,506,269, 4,364,884,3,680,130, 3,496,604, 3,396,220, 3,293,544 and 3,095,162. This inventionis particularly concerned with fused silica radomes such as aredisclosed in U.S. Pat. Nos. 3,680,130 and 3,336,873.

Fused silica is a material whose thermal shock resistance, lowdielectric constant, low loss tangent, and high temperature capabilitymake it an excellent candidate for missile radomes.

In the slip casting process, carefully controlled particle sizedistributions of fused silica powder are suspended in water and castinto plaster of paris molds. After careful drying in controlled humidityconditions the parts are then "sintered" to final density.

The difficulty in sintering fused silica is that densification occurs bya viscous flow mechanism and that the temperature required to achievesubstantial densification also causes crystallization. Thecrystallization reduces the thermal shock capabilities of the material.

The current practice is therefore to control the powder particle sizedistribution during casting such that as-cast densities of over 80% ofthe theoretical density value are obtained followed by a sintering whichonly increases density to about 85% of full density.

The drawbacks of current fused silica radome material lie primarily withthe low density, which in turn leads to low strength, low rain erosionresistance, and the need to seal the radome against moisture.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a new andimproved radome comprises a fused silica radome having a density equalto or greater than 95 percent of theoretical density.

In accordance with another aspect of the present invention, a new andimproved radome comprises a fused silica radome having a density equalto or greater than 95 percent of theoretical density, an average bendstrength equal to or greater than 10 Kpsi, a thermal expansion of about0.3×10⁻⁶ /°C., a thermal conductivity of about 1.2 BTU/hr,ft,°F., adielectric constant of about 3.8 at X-Band frequencies, a loss tangentof about 0.002 at X-Band frequencies, an average modulus of rupturegreater than 10 Kpsi from room temperature to 1000° C., and nocrystalline phase present.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 depicts curves of modulus of rupture (MOR) values as a functionof temperature of arc fused silica versus slip cast fused silicamaterials used for radomes.

FIG. 2 depicts a diagrammatical representation of the arc fusion moldingequipment for making radomes in accordance with the present invention.

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above-described drawing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention overcomes many of the mechanical propertylimitations of slip cast fused silica radomes. The arc fusion techniquerequires less particle size distribution control and employs a fusionand cooling rate sufficiently fast that no crystalline phase forms whichwould reduce the thermal shock capabilities of the material.

To perform arc fusion, a quartz or fused silica powder is poured betweena graphite mold and a mandrel and held in place by centrifugal force anda vacuum as the mold is rotated. An arc is then struck betweenelectrodes placed within the mold cavity.

The material fuses from inside to outside achieving near theoreticaldensity in the process.

To more fully illustrate this invention, the following non-limitingexample for making a 7" diameter by 20" high dense fused silica radomeis presented.

EXAMPLE

As depicted in FIG. 2, nine pounds of crystalline quartz sand (9) havinga particle size from about 20 microns to about 400 microns was pouredinto rotating graphite interior mold (8). Rotating mandrel (10) formsthe near net ID shape of the radome during the filling of rotatinggraphite interior mold (8). The rotating graphite interior mold (8) isperforated with 3/32" holes equally spaced. Graphite interior mold (8)is surrounded by porous carbon liner (2) to stop sand from being pulledinto vacuum pump (7) through pipe (6) when a vacuum of 5000 Pa isapplied throughout the fusion. The vacuum holds the sand in positionagainst graphite interior mold (8). The graphite interior mold (8),porous carbon (2), and water cooled bucket (1) cooled by means of pipe(5) all rotate by mean of rotating spindle (3) that is capable oftransferring vacuum through holes (4) in the bottom of water cooledbucket (1) between 180-210 RPM's through the fusion cycle. Three 1"diameter by 12" long graphite electrodes produce a three phase AC arcused as the heat source for fusion directly above the radome and cavity.The arc fusion parameters used to produce the 7"×20" radome are asdepicted in Table I.

                  TABLE I                                                         ______________________________________                                                               ELECTRODE TIP                                          PRO-                   TO TOP OF MOLD                                                                             VOLTAGE                                   GRAM   TIME            DISTANCE     DROP                                      STEP   (SEC)   AMPS    (INCHES)     (VOLTS)                                   ______________________________________                                        1      40      1000    8" above     150                                       2      40      1000    6" above     150                                       3      40      1000    4" above     150                                       4      40      1000    2" above     150                                       5      65      1000    0" even      150                                       6      65      1200    2" below     200                                       7      65      1200    4" below     200                                       ______________________________________                                         Total time 355 seconds                                                   

After the fusion cycle was completed the rotation was stopped andgraphite interior mold (8) with porous carbon liner (2) were immediatelypulled from the water cooled bucket (1). Graphite interior mold (8) wasmanually inverted and the near net shape radome was removed fromgraphite interior mold (8). The near net shape radome was cooled to roomtemperature in ambient conditions and was ready for interior-exteriormachining.

The mechanical properties of arc fused silica are greatly superior tothose of slip cast fused silica as depicted in Table II.

                  TABLE II                                                        ______________________________________                                         MECHANICAL PROPERTIES OF MATERIALS                                           USED FOR RADOMES                                                                             Slip Cast                                                                              Dense Arc                                                            Fused Silica                                                                           Fused Silica                                          ______________________________________                                        Density (gm/cc)  1.95       2.20                                              Bend Strength (Kpsi)                                                                           5.0        15.8                                              Young's Modulus (Mpsi)                                                                         3.5        10.5                                              Thermal Expansion                                                                              0.3        0.3                                               (× 10.sup.-6 /°C.)                                               Thermal Conductivity                                                                           0.5        1.2                                               Dielectric Constant                                                                            3.3        3.8                                               Loss Tangent     0.001      0.002                                             ______________________________________                                    

The modulus of rupture (MOR) strength of arc fused silica as a functionof temperature measured using 0.050×0.100×1.00 inch bars tested in fourpoint flexure and compared to slip cast fused silica is shown in FIG. 1.

The arc fusion process has an additional benefit when applied to radomefabrication. From an economic point of view the short duration of theradome fabrication (15 minutes for the arc fusion process of the presentinvention versus days for slip casting) leads to enhanced manufacturingflexibility.

In summary the present invention involves the utilization of arc fusionfor the fabrication of fused silica radomes to obtain superiorcharacteristics over slip cast fused silica radomes.

While there has been shown and described what is at present consideredthe preferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention as defined bythe appended claims.

What is claimed is:
 1. A silica radome comprising a fused silica radomehaving a density equal to or greater than 95 percent of theoreticaldensity.
 2. A silica radome in accordance with claim 1 wherein saidfused silica radome has an average modulus of rupture greater than 10Kpsi from room temperature to 1000° C. and no crystalline phase present.3. A silica radome in accordance with claim 1 wherein said fused silicaradome has a modulus of rupture ranging from about 20 Kpsi to about 45Kpsi a 1000° C.
 4. A silica radome in accordance with claim 1 whereinsaid fused silica radome has no crystalline phase present.
 5. A silicaradome comprising a fused silica radome having a density equal to orgreater than 95 percent of theoretical density, an average bend strengthequal to or greater than 10 Kpsi, a thermal expansion of about 0.3×10⁻⁶/°C., a thermal conductivity of about 1.2 BTU/hr,ft,°F., a dielectricconstant of about 3.8 at X-Band frequencies, a loss tangent of about0.002 at X-Band frequencies, an average modulus of rupture greater than10 Kpsi from room temperature to 1000° C., and no crystalline phasepresent.